1. Field of the Invention
The invention relates to the field of MOS integrated circuits, particularly those employing silicon on insulators.
2. Prior Art
The present application describes an improvement in a process where an epitaxial-like layer is formed over an insulative layer. The improvement of the present application results in the formation of an interconnect in the substrate.
A process for forming an epitaxial-like layer over an insulator is described in copending application, Ser. No. 700,607, filed Feb. 11, 1985, entitled PROCESS FOR FORMING ISOLATED SILICON REGIONS AND FIELD-EFFECT DEVICES ON A SILICON SUBSTRATE, assigned to the assignee of the present invention. In the process described in this application, an insulative layer is formed on a silicon substrate and openings are formed through this layer. A polysilicon layer is then deposited over the insulative layer and contacts the substrate through the openings. Various process steps are described for recrystallizing the polysilicon layer by propagating the crystalline structure of the substrate through the openings into the polysilicon layer. Relatively high quality monocrystalline silicon is formed above the seed windows through which the recrystallization occurs. These regions are used for channel regions of MOS field-effect devices. The source and drain region for these devices are formed in the recrystallized polysilicon layer adjacent to the seed windows and over the insulation; thus the source and drain regions are isolated from the substrate.
Sections of this process are described in conjunction with the present invention since the interconnect formed with the present invention is, in its currently preferred embodiment, integrated into the process described in this application.
It is well-known to form interconnects in the substrate during the fabrication of MOS devices. These interconnects are sometimes referred to as crossunders and often the dopant from a polysilicon layer is driven into the substrate to form the crossunders. Interconnects or crossunders formed in the substrate are described in U.S. Pat. Nos. 4,013,489 and 3,964,092. In these processes, the substrate itself is part of the active circuit devices. With the silicon on insulation circuits, there is an attempt to separate the active circuit from the substrate. The present invention describes a process for forming the interconnect in the substrate where the active devices themselves are formed above the insulative layer in a recrystallized layer.